(1 ) Field of the Invention
This invention is concerned with the control of an air-fuel ratio for an internal combustion engine.
(2 ) Prior Art
There are known various ways to control the air-fuel ratio in an internal combustion engine. One reason for such control is that it is possible to reduce the amount of undesirable substances in the exhaust gases of an internal combustion engine by controlling the air-fuel ratio to a figure typically near stoichiometry, which is the ratio containing fuel and oxygen in such proportions that, in perfect combustion, both would be completely consummed. It is well known that the types and amounts of substances present in the engine exhaust are greatly affected by the ratio of air to fuel in the mixture supplied to the engine. Rich mixtures with excess fuel tend to produce higher amounts of hydrocarbons and carbon monoxide. Lean mixtures with small amounts of excess air tend to produce greater amounts of oxides of nitrogen. It is also well known that those gases can be catalytically treated to reduce the amounts of these components, the catalytic treatment including oxidation of carbon monoxide and hydrocarbons and reduction of nitrogen oxides.
A typical sensor such as zirconia can sense the presence of stoichiometry, and the use of such a sensor to maintain the air-fuel ratio at stoichiometry is relatively straight forward. However, recent investigations indicate that there are occasions when it is desirable to operate lean of stoichiometry by a desired amount. It can readily be appreciated that this is a much more difficult task because typical sensors only indicate stoichiometry itself. These are some of the problems this invention overcomes.